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EXECUTIVE SUMMARY Dynamics of the Colorado River population of Colorado pikeminnow Ptychocheilus lucius, an endangered fish species endemic to the Colorado River basin, were investigated from 1991 to 2000. Two multi-year data collection efforts were made: the first, from 1991 to 1994, and the second, from 1998 to 2000. Primary objectives included capturing and marking Colorado pikeminnow from throughout the study area, developing population estimates from the resulting mark-recapture data, and assessing recruitment trends by analyzing annual length frequency histograms of the population. In addition to meeting these primary objectives, the study provided other new demographic information, some specific to the Colorado River population (adult survival rate, dispersal patterns, and trends in body condition) and some perhaps applicable to the species in general (growth rate, longevity, and sex ratio). The 286-km-long study area included the mainstem Colorado River from its confluence with the Green River upstream to the Price Stubb Diversion Dam at Palisade, Colorado and the lower 3.5 km of the Gunnison River downstream of the Redlands Diversion Dam. The 20-km long Westwater Canyon, which separates the 105.7-km upper study reach from the 180.7-km lower reach, was excluded from study; it harbors very few pikeminnow and is difficult to sample. The upper reach, from Westwater Wash to Palisade, was sampled three times annually. The lower reach, from the Green River confluence to Cottonwood Wash, was sampled twice annually. A combination of trammel-netting backwaters and electrofishing shorelines was used to capture as many Colorado pikeminnow as possible during each pass. Fish were measured, weighed, and marked with a uniquely coded passive integrated transponder (PIT) tag inserted in the body cavity. Capture data were used to develop estimates of population abundance, growth rate, dispersal patterns, body condition and length frequency. In 1999 and 2000, captured fish were sexed, and the ratio of males to females in the population was determined. Results of the 1991-1994 sampling have been previously reported; hence, this report emphasizes the results of the 1998-2000 field effort. However, all results are discussed in the context of trends over the past 10 years. Annual estimates of whole-river population size (all fish ? 250 mm TL) averaged 582 vii